Colloids in which fine particles of electroconductive substances such as metals or alloys are dispersed are generally colored intensely. A color of an object is exhibited by partial absorption of spectra of visible rays and since metal colloidal particles cause intense scatter simultaneously with absorption, coloration is remarkable. In a case of expressing the coloring property of a colloidal solution for a colloidal solution layer of a predetermined thickness by using a reciprocal of a minimal concentration at which coloration of the colloidal solution is observed, the coloring power of a gold sol is about 20,000 times as high as a divalent copper ion (Cu2+). The color of the colloidal solution is different when it is observed as a transmission light and observed as a reflection light. For example, while it appears yellow when observed through a silver sol contained in a glass container, it appears blue when a reflection light is observed. Further, coloration of the colloidal solution changes also depending on the size of the colloidal particles and, generally speaking, it shifts to a longer wavelength region as the grain size increases. The transmission color of the gold colloidal solution changes as pale crimson or red color at about 60 nm, purple at about 90 nm and blue at about 120 nm of gold particles. As the grain size increases further, the coloring property decreases since the ratio of the surface area increases. The red-purple color of the gold colloid is particularly beautiful, which is referred to as purple of Cassius. Purple of Cassius has been used long since as pigments of pink, rose, or rhodamine purple, purple color and often used for the production of red glass using glass as a supports or ceramic pigments using kaolinite. They are extremely important for providing or exhibiting red or purple color as pigment for overglaze color.
Then, gold, silver and platinum colloids have been chemically produced long since by a reducing method of corresponding salts. Among them, the gold colloid is a typical colloid embellishing the history of colloid chemistry and there have been known a number of reducing methods for obtaining gold colloidal solutions. For example, by at first neutralizing an aqueous solution of tetrachloroauric acid (H[AuCl4]) with potassium carbonate (K2CO3) and then adding formaldehyde (HCHO) as a reducing agent, a gold colloid in which fine gold particles of 8 to 9 nm are dispersed in a liquid is obtained while depending on the concentration of the initial aqueous solution of tetrachloroauric acid. The reducing reaction is considered as below.H[AuCl4]+2K2CO3+H2O→Au(OH)3+2CO2+4KCl2Au(OH)3+K2CO3→2KAuO3+3H2O+CO22KAuO2+3HCHO+K2CO3→2Au+3HCOOK+H2O+KHCO3 
The gold colloid is used again as a (seed solution) for obtaining larger gold colloidal particles. By repeating such procedures, gold colloidal particles of various sizes are prepared. Gold colloid obtained by reducing with formaldehyde is referred to as formal gold. As the reducing agent, in addition to formaldehyde, an etheric solution of yellow phosphor, hydrogen peroxide (H2O2), carbon monoxide (CO), alcohol, and, further, liquid extracts of natural products such as tea or tobacco may also be used. Depending on the condition upon formation, various products such as of red, purple or blue colors can be obtained occasionally.
Silver colloid is prepared also by substantially the same reducing method. A diluted aqueous solution of silver nitrate (AgNO3) with addition of a diluted aqueous solution of tannic acid is heated to 70 to 80° C., and a small amount of sodium carbonate (Na2CO3) is added little by little while stirring. Silver carbonate (Ag2CO3) formed in this course is reduced with tannic acid to yield metallic silver (Ag). This silver is present as a colloid in the solution and the solution exhibits a transparent brown color. In addition, according to the Carey-Lea method of obtaining a red silver colloid, precipitates of a deep blue color obtained through reduction by mixing a concentrated aqueous solution of silver nitrate with a mixed solution of citric acid and ferrous sulfate (FeSO4.7H2O) are separated by filtration and then washed with distilled water. Then, a silver colloid exhibiting a red transmission color is leached as liquid filtrates. Since the transmission color of the silver colloid is different remarkably from yellow, red or blue to green depending on the grain size and the shape of the particles thereof, the color is not always blood red even according to the Carey-Lea method and generally it is an intense red color somewhat tinted with brown. In addition, as a method of preparing the silver colloid, a method of reducing a warm aqueous solution of silver oxide (Ag2O) with carbon monoxide (CO) or hydrogen has been known. Further, as is well-known, a silver colloid dispersed in gelatin obtained by exposing a gelatin solution of silver nitrate or silver halide to light is used in photography.
Further, a platinum colloid is prepared simply by a method of adding sodium citrate to an aqueous solution of hexachloroplatinic acid (H[PtCl6]) while boiling thereby reducing the same. In addition, of using a solution of mixing hexachloroplatinic acid and tetrachloroauric acid (H[AuCl4]) at an optional ratio, a platinum-gold alloy colloid of a composition corresponding to the molar ratio thereof is obtained. Further, when an aqueous mixed solution of hexachloroplatinic acid and palladium chloride (PdCl5) is used and reduced with sodium citrate in the same manner, a platinum-palladium alloy colloid can be prepared.
While various methods contained in many research papers based on the reducing method of instable noble metal compounds such as noble metal chlorides are different in view of the difference of reducing agent, pH and temperature during reaction, or presence or absence of protective colloid, the difference is not so essential. The purity of reagents to be used and the way of handling them often give effects, and individual methods have no such meanings as capable of expecting remarkable functions and effects.
Different from the foregoing chemical methods, various physical methods have been known. For example, a noble metal colloid or noble metal alloy colloid can be obtained also by dipping two electrodes made of a starting noble metal or alloy at a slight distance in a non-electrolyte liquid such as oil or pure water filled in a container externally applied with water cooling and generating arc discharge or spark discharge therebetween. The method is referred to a Bredig method or a spark erosion method.
As another physical method, a method of preparing fine metal particles of heating to evaporate a metal element in a rare gas atmosphere at 1 to 30 Torr (130 Pa to 4 kPa), and collecting fine metal particles generated as smokes, which is referred to as an in-gas evaporation method has been known (Nobuhiko Wada: in Solid Physics, separate volume, special number, Microfine Particles, p 57 (Agne technical Center, Tokyo, 1975). The heating method includes an ohmic heating method by a tungsten resistance heater, an electron beam heating method, an arc plasma heating method, a laser heating method, etc., and while fine particles manufacturing method corresponding respectively to them have been known, they are considered basically identical with each other. It has been reported that a metal colloid is produced by collecting fine particles of various metals or alloys containing noble metals or alloys prepared by the methods described above in an organic solvent just after the generation thereof and adding an appropriate protective colloid to stably disperse them in the organic solvent, which is formulated into a conductive ink used for electronic micro circuits.
While the chemical methods of using noble metal chlorides or noble metal nitrates have been used long since as described above and versatile delivational improved methods have been proposed, it was generally difficult to obtain a concentrated colloid by the methods. Further, they have a drawback that the starting chemicals are extremely expensive, various chemical wastes are generated in the course of production process to result in large environmental load. Such a drawback resulted difficulty in producing noble metal colloids by the chemical methods in the industrial scale under economical restriction.
While the spark erosion method of using the arc discharge is a method suitable to obtain a thin colloid, it is difficult to prepare a concentrated colloid system while preventing an aggregation. This is because addition of a surfactant is indispensable for stabilizing the concentrated colloid against an aggregation, but the surfactant is generally an electrolyte and effective spark discharge cannot be obtained easily.
On the other hand, while the in-gas evaporation method of the prior art is a method having high productivity, inexpensive and industrially excellent, it is difficult to make the grain size of fine particles uniform and has a drawback that fine particles tend to be agglomerate into a cluster. This is due to the principle of the in-gas evaporation method. That is, evaporated metal atoms are cooled by collision of rare gas molecules and associated to form fine particles but the generated fine particles are again associated to each other in a rare gas molecule atmosphere and fine particles tend to form chained clusters. Bonding between fine particles is mainly due to van der Waals force. Fine particles once forming formed clusters are difficult to be dispersed in a solvent as individual fine particle system by pulverizing clusters by applying a colloid chemical process such as addition of a protective colloid.
Under the situations as described above, the inventor of the present application has already proposed a method of manufacturing a metal colloid which is referred to as a method of continuous vacuum deposition on a surface-active liquid and has already obtained a patent right (Patent Documents 1 to 4).
That is, the series of patent techniques are methods developed for the synthesis of metal magnetic fluids and, as disclosed by the inventor in the patent specification, they are applicable to the manufacture of various metal or alloy colloids including noble metal colloids.
The method is outlined as described below. As shown in FIG. 1, a vacuum vapor deposition vessel (1) is in a drum shape rotating around a fixed shaft (2) also serving as an evacuation tube. The entire drum-shaped vacuum vapor deposition vessel (1) is evacuated to a high vacuum through the fixed shaft (2). An oil (3) with addition of a surfactant is filled to the bottom of the drum and the oil containing a surfactant is drawn into a thin film on the inner wall surface of the drum as the rotation of the drum. An evaporation source (5) is fixed to the central shaft of the drum. The evaporation source (5) has a heat resistant crucible containing raw material lumps of a metal or alloy to be obtained, which is heated up to a temperature at which the metal is evaporated in the form of atoms by a tungsten resistance heater. Further, the evaporation source (5) is covered with a radiation heat shielding plate (6) for shielding the radiation while leaving the top portion. The outer wall of the rotary vacuum vessel (1) is entirely cooled by a water stream (7). Thermocouple (8) for measuring the temperature of the oil film (4) during operation is placed in contact with the inner wall of the rotary vacuum vessel (1) and move slidably along the inner wall of the vacuum vessel (1) along with rotation.
Metal atoms (9) evaporated in vacuum from the evaporation source (5) are condensed on the surface of the oil film (4) containing the surfactant to form fine metal particles (10), the generated fine metal particles (10) are transported along with rotation of the vacuum vessel (1) to the oil reservoir (3) of the vacuum vessel and, at the same time, a new oil film (4) is supplied to an upper portion of the vacuum vessel (1). By continuing the process, the oil (3) containing the surfactant at the bottom of the vacuum vessel changes into a fine metal particle colloid in which fine metal particles are dispersed at high concentration. What is important is the effect of the surfactant. Usually, when vacuum deposition is conducted on the pure oil surface, metal atoms are not deposited on the oil surface but most of them are reflected. However, in a case where the surfactant is incorporated in the oil, it is considered that the hydrophilic functional groups of the surfactant cover the surface of the oil, and the oil surface is modified so as to have depositability to the metal atoms and the metal atoms are deposited effectively. According to the experiment, about 80% or the flying metal atoms are deposited on the oil surface and condensed as fine particles. The next important effect of the surfactant is that the surface of fine particles are covered with the surfactant molecules at the same time with the formation of the fine metal particles to prevent the coagulation of fine particles to each other and growing larger, whereby the fine particles are well dispersed in the oil while keeping the size just after generation of the fine particles.
In a case of using an alloy as the evaporation source, fine alloy particles having the composition reflecting the vapor pressure of the alloy ingredients are obtained but the alloy composition in the fine particles is different greatly from the composition of the starting alloy.
By the method described above, fine particle colloids with a diameter of 2 to 3 nm are prepared for metals or alloys of a relatively high melting point such as Fe, Co, Ni, Cr, Ge, Pd, Pt, and Fe—Co alloy. On the other hand, fine particle colloids with a diameter of from 5 to 9 nm are prepared for metals and alloys of low melting point such as Zn, Cu, Ag, Au, In, and Sn.
As has been described above, the method of the continuous vacuum deposition on a surface-active liquid is a method capable of efficiently producing colloids containing fine metal particles with the smallest grain size at high concentration, without the agglomeration and in a simple process.
Further, since the yield of the starting metal is high, it is excellent also in economic efficiency as a method of preparing expensive noble metal colloids.    Patent Document 1: JP No. 1374264    Patent Document 2: JP No. 1348706    Patent Document 3: JP No. 1716879    Patent Document 4: JP No. 1725153